Karyotypes – more than just a pretty picture

Posted
2 years ago
by Gwen Wathne
•

6 min read

Thanks to our supporters, we can fund some pretty amazing research. We love talking scientific research and how each project helps tackle cancer, but some research concepts are easier to explain than others.

One topic that I’ve found more difficult to describe is something called chromosomal translocation. This is where part of one chromosome has attached to another, to form a new, abnormal chromosome. This abnormal chromosome has genetic information from both original chromosomes, and together, they create new genes or proteins, some of which can cause cancer to develop.

Here’s a breakdown of the topic to give a better understanding of the idea and how these colourful images, called ‘karyotypes’, are being used to help diagnose diseases such as those that lead to blood cancers, including a project we funded back in 1999.

Karyo-what?

Healthy human cells have 46 chromosomes, organised into 23 pairs, as illustrated in the image above . All our genetic information is stored within these chromosomes as DNA. Some chromosomes are bigger than others, and they have different shapes. The picture above shows a karyotype. It is a way to organise and count chromosomes, to check that everything is as it should be.

Depending on how long ago you were at school, and if you studied Biology, you may have seen a black and white version of this in a textbook. Modern technology now allows scientists to see it in colour by using fluorescently-labelled probes. In this case, the probe is a fluorescent copy of the DNA, which acts like a magnet to attach itself to the DNA in each chromosome, so that we can see this multi-coloured version, called a spectral karyotype. Using different colours to be able to tell chromosomes apart, makes it easier to see when abnormal chromosomes have been created. The abnormal chromosomes show up with two or more colours, where normal chromosomes will only be one colour. In the picture, you can see that one of the chromosomes in pair number three is smaller than the other, and part of this chromosome can now be seen on one of the chromosomes from pair 9. Can you spot any other differences in the pairs or ‘translocations’?

Sometimes this type of karyotype is used to help diagnose a group of blood disorders, called myeloproliferative disorders (MPD), such as different types of leukaemia.

How have Worldwide Cancer Research helped?

In 1999 we gave funding to Dr Berna Beverloo and Dr Rosalyn Slater at Erasmus MC in Rotterdam, The Netherlands, who wanted to use spectral karyotyping to find new chromosome changes in blood cancer such as acute myeloid leukaemia (AML) and acute lymphoblastic leukaemia (ALL), which had not been found identified with other techniques. They were the first to show the involvement of the JARID1A (now renamed KMD5A) gene in leukaemia, through its fusion with theNUP98gene.

More recently, this work was taken further, and researchers found that this NUP98-JARID1A fusion is a commonly occurring genetic abnormality in a type of childhood leukaemia called acute megakaryoblastic leukaemia (AMKL).

Chromosomal translocation and the BCR-ABL gene

Nearly all patients who have a type of leukaemia called chronic myeloid leukaemia (CML) have an abnormal chromosome called the Philadelphia chromosome, formed by chromosomal translocation. The Philadelphia chromosome produces a gene called BCR-ABL, which makes the signals that cause leukaemia cells to grow and divide. Doctors often test for the Philadelphia chromosome when they are trying to diagnose CML.

But some MPD patients do not have the Philadelphia chromosome. One such disease is called Essential thrombocythaemia (ET), a rare disease affecting our blood cells, which can sometimes develop into a type of leukaemia called AML.

Before starting their Worldwide Cancer Research grant back in 1999, Dr Adina Aviram and her team at the Rabin Medical Center in Israel had identified a group of patients with ET who did not have the Philadelphia chromosome but somehow still had the abnormal BCR-ABL gene. They thought that the presence of the BCR-ABL gene in MPDs other than CML might be important in understanding the history of MPD, and the changes that happens in a person’s body because of MPD.

They used their grant to study specific changes to DNA, called methylation, in these patients, in the hope of shedding some light on the early events involved in the chromosomal translocation that creates the abnormal BCR-ABL gene. Their results helped to add to the growing pool of knowledge that there may be another MPD, separate from CML and ET. This has not yet been confirmed, but doctors now know that they need to check for BCR-ABL in patients who do not have the Philadelphia chromosome when they are trying to diagnose MPDs.

What impact did our research make on cancer?

When we contacted Dr Aviram to find out where the results from their Worldwide Cancer Research grant had led to after the grant ended, she told us “I think our study together with studies from other laboratories helped to change the routine diagnosis of MPD.”

Even now, many years after their initial findings, Dr Aviram is still getting questions about the study, which shows just how important this work has been for the medical and scientific community.